// Textbook fragment 07.10
public static <E> String parentheticRepresentation(Tree<E> T, Position<E> v) {
  String s = v.element().toString(); // main visit action
  if (T.isInternal(v)) {
    Boolean firstTime = true;
    for (Position<E> w : T.children(v))
      if (firstTime) {
        s += " ( " + parentheticRepresentation(T, w);  // the first child
        firstTime = false;
      }
      else s += ", " + parentheticRepresentation(T, w); // subsequent child
    s += " )"; // close parenthesis
  }
  return s;
}